Gallium nitride field effect transistor with buried field plate protected lateral channel

ABSTRACT

A method for fabricating a lateral gallium nitride (GaN) field-effect transistor includes forming a first and second GaN layer coupled to a substrate, removing a first portion of the second GaN layer to expose a portion of the first GaN layer, and forming a third GaN layer coupled to the second GaN layer and the exposed portion of the first GaN layer. The method also includes removing a portion of the third GaN layer to expose a portion of the second GaN layer, forming a source structure coupled to the third GaN layer. A first portion of the second GaN layer is disposed between the source structure and the second GaN layer. A drain structure is formed that is coupled to the third GaN layer or alternatively to the substrate. The method also includes forming a gate structure coupled to the third GaN layer such that a second portion of the third GaN layer is disposed between the gate structure and the second GaN layer.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/077,039, filed on Nov. 11, 2013, the disclosure of which isincorporated by reference herein in its entirety for all purposes.

BACKGROUND OF THE INVENTION

Power electronics are widely used in a variety of applications. Powerelectronic devices are commonly used in circuits to modify the form ofelectrical energy, for example, from AC to DC, from one voltage level toanother, or in some other way. Such devices can operate over a widerange of power levels, from milliwatts in mobile devices to hundreds ofmegawatts in a high voltage power transmission system. Despite theprogress made in power electronics, there is a need in the art forimproved electronics systems and methods of operating the same.

SUMMARY OF THE INVENTION

The present invention relates generally to electronic devices. Morespecifically, the present invention relates to providing a buried fieldplate in a lateral channel high voltage device made with high band gapmaterials. Merely by way of example, the invention has been applied tomethods and systems for manufacturing field-effect transistors (FETs)using gallium nitride (GaN) (and/or other III-V materials), as well assilicon carbide (SiC). In fact, the methods and techniques providedherein can be applied to a variety of FET devices such as the junctionfield effect transistor (JFET), metal-semiconductor field effecttransistor (MESFET), high-electron-mobility transistor (HEMT), and/orother such structures. Within this disclosure we recognize that all FETdevices have a common structure with common function blocks that stemsfrom the principle of field-effect control of the current flow.

Without limiting embodiments of the present invention, without anattempt to provide a complete specification of the field-effecttransistor, and considering only an idealized device structure withoutvarious secondary mechanisms and events, the major components of a fieldeffect transistor: source, drain, gate, substrate, channel anddrift-region. The charge carriers forming the device current, which iscontrolled by the FET, enters the FET from the source and leaves fromthe drain. The device current is controlled by the gate, which dependingon the potential applied the gate, creates a field profile that affectsthe desired outcome of controlling the current to a desired level. Thegate lies in close physical proximity to the channel. The channel is theregion of the device where the majority of the field effect control ofthe device current is achieved. The drift-region is the region of thedevice that carries the device current from the channel region to thedrain. The drift region sustains the majority of the potential dropacross the device when the device is in non-conducting mode under highdrain bias. The substrate is the physical structure on which the othercomponents are located. The methods and techniques provided herein canbe applied to a variety of devices deriving functionality from a subsetof the components of a FET device. For example, for an insulated gateFET (IGFET), the gate can be used as a one of the plates of high voltagecapacitor. For a MESFET, the gate, channel, drift region and drain canbe used to form a high voltage rectifier.

Within the description of the present process flows, it should beappreciated that the channel and the drift-region can be manufactured tolie along and conduct current along either the lateral or verticaldirection. In this context, the lateral direction is understood to bethe direction parallel to the planar surface of the substrate and thevertical direction is understood to be the directionperpendicular/normal to the planar surface of the substrate. Embodimentsof the present invention provide a buried field plate for lateralchannel FET devices that can incorporate either a lateral or verticaldrift region.

According to an embodiment of the present invention, a method forfabricating a lateral channel GaN field-effect transistor (FET)protected by a buried field plate is provided. The method includesproviding a substrate and forming a first GaN layer coupled to thesubstrate. The first GaN layer has a first conductivity type. The methodfurther includes forming a second GaN layer coupled to the first layer.The second layer has a second conductivity type. The method furtherincludes removing a first portion of the second GaN layer to expose aportion of the first GaN layer and forming a third GaN layer coupled tothe second GaN layer and the exposed portion of the first GaN layer. Thethird GaN layer has a third conductivity type. Depending on the desiredfunctionality, the method may further include removing a portion of thethird GaN layer to expose a portion of the second GaN layer and formingan electrical contact to the second GaN layer.

In this embodiment, the second GaN layer functions as buriedfield-plate. To attain the function of a buried field plate andproviding protection to the gate, the conductivity type of the secondGaN layer is complementary to the conductivity type of third GaN layer.The method may also include forming a source structure coupled to thethird GaN layer such that a first portion of the third GaN layer isdisposed between the source structure and the second GaN layer.

In one implementation, the method also includes forming a gate structurecoupled to the third GaN layer such that a second portion of the thirdGaN layer is disposed between the gate structure and the second GaNlayer. This second portion of the third GaN layer forms the channel. Assuch, the device current is carried by carriers of the same type as theconductivity type of the third GaN layer. The method also includesforming a drain structure.

For a vertical-drift region FET, the drain can be formed as coupled tothe substrate. In this case, the substrate is conductive and thesubstrate, the first GaN layer and third GaN layer are of the sameconductivity type. The conductivity type of the second GaN layer iscomplementary to third conductivity type.

For a lateral-drift region FET, the drain can be coupled to the thirdGaN layer and is positioned such that the gate rests between the sourceand drain. In this case, the conductivity types of the third GaN layerand the substrate can be different from the conductivity type of thethird GaN layer and can be set to be desired properties of thesubstrate. The conductivity type of the second GaN layer iscomplementary to third conductivity type.

According to a particular embodiment of the present invention, alateral-drift field-effect transistor (FET) is provided. Thelateral-drift FET, also referred to as a lateral-drift region FET,include a substrate, a first GaN-based layer coupled to the substrate,and a second GaN-based layer coupled to the first GaN-based layer. Thelateral-drift FET also includes a third GaN-based layer coupled to thesecond GaN-based layer and a portion of the first GaN-based layer and achannel region disposed in the third GaN-based layer. The lateral-driftFET further comprises a source structure coupled to the third GaN-basedlayer, a gate structure coupled to the third GaN-based layer, and adrain structure coupled to the third GaN-based layer.

The lateral-drift FET can further include a contact structure coupled tothe second GaN-based layer. A portion of the second GaN-based layer canform a field plate structure. The field plate structure can have aplurality of sections that are progressively thinner as a distance fromthe source structure increases. A first portion of the channel regiondisposed between the source structure and the second GaN-based layer canbe thinner than a second portion of the channel region disposed betweenthe gate structure and the second GaN-based layer.

According to another particular embodiment of the present invention, avertical-drift field-effect transistor (FET), also referred to as avertical-drift region FET is provided. The vertical-drift FET includes asubstrate, a first GaN-based layer coupled to the substrate, and asecond GaN-based layer coupled to the first GaN-based layer. Thevertical-drift FET also includes a third GaN-based layer coupled to thesecond GaN-based layer and a portion of the first GaN-based layer and achannel region disposed in the third GaN-based layer. The vertical-driftFET further includes a source structure coupled to the third GaN-basedlayer, a gate structure coupled to the third GaN-based layer, and adrain structure coupled to the substrate.

The vertical-drift FET can also include a contact structure coupled tothe second GaN-based layer. A portion of the second GaN-based layerforms a field plate structure, which can have a plurality of sectionsthat are progressively thinner as a distance from the source structureincreases. A first portion of the channel region disposed between thesource structure and the second GaN-based layer is thinner than a secondportion of the channel region disposed between the gate structure andthe second GaN-based layer.

Numerous benefits are achieved by way of the present invention overconventional techniques. For example, the usage of a buried field plateas described herein can result in improvements in both deviceperformance limits and reliability. By avoiding the use of a dielectricin the field plate, the device will not be subject to certainshortcomings of a dielectric, such as time-dependent dielectricbreakdown (TDDB). Additionally, there can be less trapping and/or otherinterface issues. These and other embodiments of the invention, alongwith many of its advantages and features, are described in more detailin conjunction with the text below and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is simplified cross-sectional diagram illustrating asemiconductor device according to an embodiment;

FIG. 1B is simplified cross-sectional diagram illustrating asemiconductor device according to another embodiment;

FIGS. 2A-2E are simplified cross-sectional diagrams illustrating aprocess for creating a vertical GaN field-effect transistor (FET),according to one embodiment;

FIG. 3A-3E are simplified cross-sectional diagrams illustrating aprocess for creating a vertical GaN FET, according to anotherembodiment; and

FIG. 4 is simplified flowchart illustrating an embodiment of a method offabricating a vertical GaN FET with a buried field plate.

In the appended figures, similar components and/or features may have thesame reference label. Further, various components of the same type maybe distinguished by following the reference label by a dash and a secondlabel that distinguishes among the similar components. If only the firstreference label is used in the specification, the description isapplicable to any one of the similar components having the same firstreference label irrespective of the second reference label.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The present invention relates generally to electronic devices. Morespecifically, the present invention relates to providing a buried gateand field plate in a vertical high voltage device made with high bandgap materials. Merely by way of example, the invention has been appliedto methods and systems for manufacturing field-effect transistors (FETs)using gallium nitride (GaN) (and/or other III-V materials), as well assilicon carbide (SiC). Furthermore, these methods and techniquesprovided herein can be applied to a variety of vertical and lateralFETs, such as the JFET, MESFET, HEMT, and/or other such structures.Additional description related to field plates is provided in U.S.patent application Ser. No. 13/300,028, entitled “GaN-Based SchottkyBarrier Diode with Field Plate,” filed on Nov. 18, 2011, the disclosureof which is hereby incorporated by reference in their entirety for allpurposes

GaN-based electronic and optoelectronic devices are undergoing rapiddevelopment, and are expected to outperform competitors in silicon (Si)and other materials. Desirable properties associated with GaN andrelated alloys and heterostructures include high band gap energy forvisible and ultraviolet light emission, favorable transport properties(e.g., high electron mobility and saturation velocity), a high breakdownfield, and high thermal conductivity. In particular, electron mobility,μ, is higher than competing materials for a given doping level, N. Thisprovides low resistivity, p, because resistivity is inverselyproportional to electron mobility, as provided by equation (1):

$\begin{matrix}{{\rho = \frac{1}{q\; \mu \; N}},} & (1)\end{matrix}$

where q is the elementary charge.

Another superior property provided by GaN materials, includinghomoepitaxial GaN layers on bulk GaN substrates, is high criticalelectric field for avalanche breakdown. A high critical electric fieldallows a larger voltage to be supported over smaller length, L, than amaterial with a lower critical electric field. A smaller length forcurrent to flow together with low resistivity give rise to a lowerresistance, R, than other materials, since resistance can be determinedby equation (2):

$\begin{matrix}{{R = \frac{\rho \; L}{A}},} & (2)\end{matrix}$

where A is the cross-sectional area of the channel or current path.

Homoepitaxial GaN layers on bulk GaN substrates also have relatively lowdefect density compared to materials grown on mismatched substrates,such as GaN grown on silicon, silicon carbide (SiC), or sapphire.Homoepitaxial GaN layers on bulk GaN substrates therefore have largeminority carrier lifetime in intrinsic and/or low-doped regions ofsemiconductor devices that use these materials, enhancing the carrierinjection effect for wider base regions. The low defect density alsogives rise to superior thermal conductivity.

Embodiments of the present invention incorporate a buried back gate andfield plate in structures utilizing GaN and/or other high band gapmaterials. For lateral channel high-voltage devices such as the JFET,MESFET or HEMT, control of the electric field along the lateraldirection between the gate and drain can be critical for performance andreliability. To achieve the desired field profile control, Si devicessuch as laterally-diffused metal oxide semiconductor (LDMOS)transistors, employ modulation of the doping along the lateraldirection. For such lateral doping modulation, microfabrication toolssuch as diffusion or ion-implantation are used. However for GaN inparticular, but also for other large band gap materials such as SiC,these microfabrication techniques are not available or feasible.Alternatively, such tools may involve difficult techniques or processesdetrimental to other device characteristics. Hence, the lateral fieldcontrol in GaN and similar material systems is best provided by afield-plate.

In all its various forms and shapes that have been discussed in theliterature, the field plate structure typically consists of a metallicconducting structure placed on top of a dielectric. Although the lateralfield is reduced, the field plate can still have a significantdisadvantage because the lateral high field can occur close to thesurface along the dielectric-GaN interface. Also a high vertical highfield can occur across the dielectric. In the presence of high lateraland vertical fields on the dielectric, the quality and properties of thedielectric-GaN interface and the dielectric itself becomes a criticalfactor of device performance limits and reliability. Given the variousfundamental and practical constraints on the quality and properties ofthe available dielectrics that can be employed on GaN in particular butalso for other large band gap materials, the utility and usefulness ofthe field-plate method of controlling the lateral field is greatlyreduced with the resulting effect that the lateral GaN devices are leftat a disadvantage and unable to achieve the theoretical maximumfigure-of-merits given by the inherent internal properties of GaN.

Embodiments of the present invention provide a new approach forcontrolling the lateral field in lateral-channel vertical or lateraldrift-region devices using GaN and/or other materials. Rather than usinga metallic field-plate acting through a dielectric, embodiments utilizea non-insulating structure for field control. This approach can beapplied to wide band gap materials such as GaN and/or SiC, as well asother materials. Although embodiments herein describe the use of GaN,embodiments may utilize any other material system where both p and ntype doping is available, including SiC and other wide band gapmaterials.

FIG. 1A is a cross-sectional illustration of alateral-channel/lateral-drift region FET 100 according to oneembodiment. Components of this embodiment include a substrate 115, firstGaN layer 110, field plate structure 120, field plate contact 123,channel region 130, gate contact 160, source contact 140, and draincontact 150. The structure and composition of these components can vary,depending on desired functionality. For example, depending on thespecific device structure, the channel region 130 can incorporaten-type, p-type semiconductors, various heterostructures, and/or variousvertical doping profiles. A person of ordinary skill in the art willrecognize many additions, substitutions, and other variations.

FIG. 1B is a cross-sectional illustration of alateral-channel/vertical-drift region FET 101 according to oneembodiment. Components of this embodiment include a substrate 116, afirst GaN layer 111, field plate structure 121, field plate contact 124,channel region 131, gate contact 161, source contact 141, and draincontact 151. The structure and composition of these components can vary,depending on desired functionality. For example, depending on thespecific device structure, the channel region 131 can incorporaten-type, p-type semiconductors, various heterostructures, and/or variousvertical doping profiles. A person of ordinary skill in the art willrecognize many additions, substitutions, and other variations

Operation of the lateral-drift FET 100 can vary, depending on desiredfunctionality. In general, current can flow from the source contact 140to the drain contact 150 in the direction indicated by the arrow 170 and190. In a normally-on configuration, the lateral-drift FET 100 can beturned off by applying a bias to the gate contact 160 and/or field platecontact 123 to deplete a region 180 between the gate contact 160 and thefield plate structure 120 and cutoff the current. Some configurationsmay provide normally-off functionality in which the region 180 betweenthe gate contact 160 and the field plate structure 120 is normallydepleted, and the device is turned on by applying a bias to the gatecontact 160 and/or field plate contact 123. In some embodiments, thegate contact 160 and field plate contact 123 may be electricallyconnected so that the field plate structure 120 is given the same biasas the gate contact 160. In other embodiments, the field plate structure120 can be given the same bias as the source contact 140. Alternatively,the field plate contact 123 may be biased separately. The properties ofthe field plate structure 120 can change depending on the way the fieldplate structure 120 is biased, and as such, the biasing connection ofthe field plate structure 120 can be set to control, for example, thecapacitance characteristics of the lateral-drift FET 100.

Operation of the vertical-drift FET 101 can vary, depending on desiredfunctionality. In general, current can flow from the source contact 141to the drain contact 151 in the direction indicated by the arrows 171and 191. In a normally-on configuration, the vertical-drift FET 101 canbe turned off by applying a bias to the gate contact 161 and/or fieldplate contact 124 to deplete a region 181 between the gate contact 161and the field plate structure 121 and cutoff the current. Someconfigurations may provide normally-off functionality in which theregion 181 between the gate contact 161 and the field plate structure121 is normally depleted, and the device is turned on by applying a biasto the gate contact 161 and/or field plate contact 124. In someembodiments, the gate contact 161 and field plate contact 124 may beelectrically connected so that the field plate structure 121 is giventhe same bias as the gate contact 161. In other embodiments, the fieldplate structure 121 can be given the same bias as the source contact141. Alternatively, the field plate contact 124 may be biasedseparately. The properties of the field plate structure 121 can changedepending on the way the field plate structure 121 is biased, and assuch, the biasing connection of the field plate structure 121 can be setto control, for example, the capacitance characteristics of thevertical-drift FET 101.

Referring to FIGS. 1A and 1B, the field plate structure 120/121 canextend laterally beyond the gate contact 160/161 to provide field platefunctionality. Moreover, the field plate structure 120/121 can bepatterned in various ways to achieve a detailed control of thefield-profile between the gate contact 160/161 and the drain contact150/151 so that the peak lateral field is reduced and the potential dropalong the channel is achieved gradually and smoothly to avoid sharppeaks in the resulting field profile. In the embodiment illustrated inFIG. 1, for example, sections of a portion 125/126 of the field platestructure 120/121 extending laterally beyond the gate contact 160/161are successively reduced in thickness. In such an embodiment, thethinnest section at the outermost part of the field plate structure120/121 may deplete in a controlled fashion under high drain bias. Thiscan prevent possible detrimental effects on the breakdown voltage. Insome embodiments, such as embodiments that integrate an avalanche diodeto the transistor structure, the outermost part of the field platestructure 120/121 can be so that a desired breakdown into the structure120/121 is achieved at a desired potential.

FIGS. 2A-2E are simplified cross-sectional diagrams illustrating aprocess flow for fabrication of a FET device according to a firstembodiment of the present invention. Although the example provided usesGaN materials, other embodiments may utilize additional and/oralternative materials and/or structures, which may be formed usingprocesses other than those described in relation to FIGS. 2A-2E.

Referring to FIG. 2A, a first GaN layer 205 is formed on the substrate200. The substrate 200 can comprise any of a wide variety of materials,depending on desired functionality. In some embodiments, the substrate200 can be a pseudo-bulk GaN material and/or other material on which thefirst GaN layer 205 is epitaxially grown. Dopant concentrations (e.g.,doping density) and thickness of the substrate 200 can vary, dependingon, for example, whether the substrate 200 will conduct any current (asfor example in the case of a vertical-drift FET). In some embodimentsfor a vertical-drift FET, a substrate 200 can comprise an n conductivitytype, with dopant concentrations ranging from 1×10¹⁵ cm⁻³ to 1×10¹⁹cm⁻³. Although the substrate 200 is illustrated as including a singlematerial composition, multiple layers can be provided as part of thesubstrate. Moreover, adhesion, buffer, and other layers (notillustrated) can be utilized during the epitaxial growth process thatwould be used for the fabrication of a substrate that includes multiplelayers. One of ordinary skill in the art would recognize manyvariations, modifications, and alternatives.

The properties of the first GaN layer 205 depend on the configuration ofthe FET. For a vertical-drift region FET the first GaN layer 205 formsthe drift region. As mentioned earlier for a vertical-drift region FET,the conductivity type of the substrate, first and third GaN layers isthe same. The doping and thickness of the first GaN layer can be variedas a function of distance from the substrate and can be adjusted toaccommodate a given potential drop. In an example embodiment, the firstGaN layer 205 can have a n-conductivity type, with dopant concentrationsbetween about 1×10¹⁵ cm⁻³ to 1×10¹⁷ cm⁻³.

The properties of the second GaN layer 210 can also vary, depending ondesired functionality. The second GaN layer 210 can ultimately serve asa field plate and/or back gate for the vertical GaN FET, and thereforecan be doped accordingly to help ensure a properly-sized depletionregion during on and off states. In an example embodiment, the secondGaN layer 210 can have a p conductivity type, with dopant concentrationsbetween about 1×10¹⁵ cm⁻³ to 1×10²⁰ cm⁻³.

Different dopants can be used to create n- and p-type GaN layers andstructures disclosed herein. For example, n-type dopants can includesilicon, germanium, oxygen, or the like. P-type dopants can includemagnesium, beryllium, zinc, or the like.

The thickness 215 of the second GaN layer 210 can also varysubstantially, depending on doping levels, desired breakdown voltage,and/or other factors. Where subsequent etching or other removalprocesses are used, it can be beneficial to make thickness 215 of thesecond GaN layer at least as thick as the thickest portion of thedesired field plate. This enables a manufacturer to simply removeportions of the second GaN layer 210 to form the desired profile of thefield plate. In some embodiments, the thickness 215 can be between 0.1μm and 0.4 μm, for example. In other embodiments, thickness 215 can begreater than 0.4 μm or less than 0.1 μm.

Referring to FIG. 2B, a portion 220 of the second GaN layer 210 isremoved to expose a portion of the first GaN layer 205. The positioningof the exposed first GaN layer 205 can vary, depending on desiredbreakdown voltages and/or other factors. For example, the exposed firstGaN layer 205 will lie below the subsequently-formed drain contact,while the remaining portion of the second GaN layer 210 will lie belowthe subsequently-formed gate contact. The distance between the gatecontact and the edge of the field-plate can determine the breakdownvoltage of the lateral GaN JFET. When being employed as a field plate,the doping, thickness, length, and patterning of the portion of thesecond GaN layer 210 extending beyond gate towards the drain contact(see portion 275 of FIG. 2E) can be selected to improve the breakdownvoltage. The removal process can include a patterning and etchingprocess. In some embodiments, the use of epitaxial regrowth and growthmasks to form one or more of the layers and/or structures describedherein may be used additionally or alternatively to layer formationfollowed by patterning and etching.

Referring to FIG. 2C, a third GaN layer 230 is then formed, coupled tothe remaining portion of the second GaN layer 210 and the exposed firstGaN layer 205. The third GaN layer 230 forms an active region of thedevice. More specifically, the third GaN layer 230 forms a channelregion through which current flows through the FET. The third GaN layer230 can include various layered structures that would aid the formationof the device contact structures in latter fabrication steps, forexample, through patterning and etching of a portion of the 230 inselected areas. The third GaN layer 230 may be formed using any of avariety of techniques, including epitaxial regrowth.

Attributes of the third GaN layer 230 can vary depending on desiredfunctionality. Where the conductivity of the third GaN layer 230 ischosen to form a p-n junction with the second GaN layer 210. Forinstance, if the second GaN layer 210 is formed from a p-type GaNmaterial, the third GaN layer 230 will be formed from an n-type GaNmaterial, and vice versa. Furthermore, the third GaN layer 230 can bedoped to help ensure desired conductivity and a properly-sized depletionregion during on and off states. In an example embodiment, the third GaNlayer 230 can have an n conductivity type, with dopant concentrationsbetween about 1×10¹⁵ cm⁻³ to 1×10¹⁸ cm⁻³. Thickness 235 of the channelregion of the third GaN layer 230 (that is, the region above theremaining portion of the second GaN layer 210), can also vary, dependingon desired functionality. In some embodiments, the thickness 235 of thechannel region may be 1 μm or less, although thickness larger than 1 μmmay be utilized in other embodiments. In further example embodiments thethird GaN layer can also incorporate a heterostructure so that a highlyconductive 2 dimensional electron gas is formed.

Referring to FIG. 2D, a portion 240 of the third GaN layer 230 above thefirst GaN layer 210 is removed to expose a portion of the second GaNlayer 210. The removal process can include a patterning and etchingprocess.

Referring to FIG. 2E, various contact structures are formed on thesecond GaN layer 210 and third GaN layer 230, including the field platecontact structure 250, source contact structure 260, gate contactstructure 270, and drain contact structure 280. Contact structures maycomprise any of a variety of materials and may form Ohmic and/orSchottky contacts, based on desired functionality. The fabricationprocess of the contacts structures may include regrowth, selective arearegrowth or patterning and etching of the layer 230. In someembodiments, the contact structures can be formed from GaN (e.g., p-typeGaN if the third GaN layer 230 is n-type GaN) using epitaxial regrowthor etching. That said, the materials used to form some contactstructures may be different than those to form other contact structures.For example, the gate contact structure 270 in different devicestructures can incorporate metals, pGaN, heterostructures, and/ordielectrics, which may differ from the materials used in the othercontacts. Various and detailed techniques to manufacture thesestructures are well-known to workers experienced in the art.

Referring to FIG. 2E, the drain contact 280 can be alternatively placedon the substrate to form a vertical-drift FET. For this case, theconduction properties of the first GaN layer and the substrate aredesigned appropriately as described earlier.

The gate contact structure 270 is formed over the remaining portion ofthe second GaN layer 210 to allow the current in the channel portion ofthe third GaN layer 230 to be pinched off between the second GaN layer210 and the gate contact structure 270 when bias is applied to the gatecontact structure 270 and/or the field plate contact structure 250.Moreover, the remaining portion of the second GaN layer 210 extendslaterally beyond the gate contact structure 270 to form a field platefor the device. The lateral distance 275 beyond the edge of the gatecontact structure 270 that the remaining portion of the second GaN layer210 extends can vary, depending on physical features of the device(e.g., distance between the gate contact structure 270 and the draincontact structure 280) and other factors. In some embodiments, thisdistance 275 can be between 0.25 μm and 5 μm. As an example, thedistance can be between 1 μm and 5 μm. Other embodiments may includedistances outside this range.

FIGS. 3A-3E are simplified cross-sectional diagrams illustrating aprocess flow for fabrication of a FET device according to a secondembodiment of the present invention. In general, the steps of FIGS.3A-3E echo the steps shown in FIGS. 2A-2E, as described above, withcomponents 300-390 corresponding to similar components 200-290. In theembodiment of FIGS. 3A-3E, however, the remaining portion of the secondGaN layer 310 is patterned (as shown by removed portions 320 in FIG.3B), causing the lateral profiles of the second GaN layer 310 and thirdGaN layer 330 to be different. The remaining portion of the second GaNlayer 310 can be patterned in various ways to achieve a detailed controlof the field-profile between the gate and drain so that the peak lateralfield is reduced and the potential drop along the channel is achievedgradually and smoothly. The patterning can include lithography andetching and/or may involve several masked regrowth steps if necessitatedby other constraints.

As shown in FIG. 3E, the second GaN layer 310 includes a notched portion317 below the source contact 360, and a field plate portion 313extending from below the gate contact 370 laterally toward the draincontact 380. The notched portion 317 can allow for a larger channelregion under the source contact 360, which can reduce source-to-gateaccess resistance. Thicknesses of these portions can vary, and maydepend on operating voltage, doping profiles, and/or other factors. Inan example embodiment, the thickness of the notched portion is 0.1 μm,and thicknesses of the field plate portion 313 range from 0.5 μm to 0.1μm. Other embodiments can have other thicknesses.

Embodiments can incorporate numerous variations to the example shown inFIGS. 3A-3E. Thicknesses can vary from those described in the exampleabove. Furthermore, although the field plate portion 313 illustrated hasthree sections that are progressively thinner as the distance from thesource contact 360 increases, embodiments may include more or fewersections. A person of ordinary skill in the art will recognize manyadditions, substitutions, and/or other variations.

The processes shown in FIGS. 2A-2E and 3A-3E are provided as examplesonly, and are not limiting. Furthermore, the figures are not drawn toscale, and the manufacture of the FET can include additional stepsand/or features that are not shown. For example, a drain contact may becreated on the bottom of substrates 200 and/or 300 (additionally oralternatively to the drain contacts 280, 380 illustrated) to providevertical current flow. The layers and/or structures described herein mayinclude a number of sublayers, substructures, and/or other components toprovide the functionality described herein. One of ordinary skill in theart would recognize many variations, modifications, and alternatives.

FIG. 4 is simplified flowchart illustrating an example method offabricating either a lateral-drift GaN FET or a vertical-drift GaN FET,such the FETS of the embodiments previously described. The method caninclude providing a substrate (410). The substrate can conductivity canbe adjusted to provide the desired properties. The substrate can includeone or more layers and/or materials (such as GaN), including compositestructures of multiple layers, on which subsequent layers may be formed.One of ordinary skill in the art would recognize many variations,modifications, and alternatives. As discussed above, suitable doping andthickness can be chosen for embodiments in which device current flowsthrough one of more of the GaN layers described below and the substrate.

The method further includes forming a first GaN layer coupled to thesubstrate and a second GaN layer coupled to the first GaN layer (420).The field plate and/or back gate structure(s) can be formed from thesecond GaN layer. Thus, properties of the second GaN layer (e.g., dopingand thickness) can be adjusted accordingly.

Epitaxial regrowth can be used, for example, to form the first and/orsecond GaN layers. One method of forming the first and/or second GaNlayers (and/or other GaN layers described herein) can be through aregrowth process that uses an in-situ etch and diffusion preparationprocesses. These preparation processes are described in U.S. patentapplication Ser. No. 13/198,666, filed Aug. 4, 2011, entitled “Methodand System for Formation of P-N Junctions in Gallium Nitride BasedElectronics,” the entirety of which is hereby incorporated by reference.

The method further includes removing a portion of the second GaN layerto expose a portion of the first GaN layer (430). This can involve apatterning and etch process, which may include additional steps ofdepositing, patterning, and removing photoresist and/or an etch mask.The location of the removed portion of the second GaN layer can varydepending on the location of subsequently-formed gate and drainstructures, as previously described. In some embodiments, the lateralprofiles of the first and/or second GaN layers can be altered throughadditional regrowth and/or etching steps to provide a desired lateralfield profile for the device.

The method also includes forming a third GaN layer coupled to the secondGaN layer and the exposed portion of the first GaN layer (440). Asindicated previously, the third GaN layer can have the oppositeconductivity type as the second GaN layer, thereby allowing a depletionregion to be formed at the interface of the two layers. The channelregion of the FET can be formed from the third GaN layer. Thus,properties such as doping and thickness of the third GaN layer can beprovided accordingly. A portion of the third GaN layer can be removed toexpose a portion of the second GaN layer (450), enabling a field platecontact to be subsequently formed on the exposed portion of the secondGaN layer (460). In some embodiments, however, the field plate may befloating (i.e., not tied to a voltage source). In such cases, a fieldplate contact may not be created and/or used.

A source structure is then formed, coupled to the third GaN layer suchthat a first portion of the third GaN layer is disposed between thesource structure and the second GaN layer (470). For a lateral-driftFET, a drain structure is formed, coupled to the third GaN layer and fora vertical-drift FET, a drain structure is formed, coupled to thesubstrate (480). For an IGFET, source and drain structures can be formedincorporating GaN of the opposite conductivity type as the third GaNlayer. If the third GaN layer is n-type GaN, the source and drainstructures can be p-type GaN, and vice versa.

A gate structure is then formed, coupled to the third GaN layer suchthat a second portion of the third GaN layer is disposed between thegate structure and the second GaN layer (490). As indicated above, thegate structure can incorporate metals, pGaN, heterostructures, and/ordielectrics. This enables current flow between the source and drainstructures to be controlled by biasing the gate structure and/or secondGaN layer. Additionally, the second GaN layer can extend laterallybeyond the gate structure toward the drain structure, enabling a portionof the second GaN layer to act as a field plate for the device. Spacingbetween the gate structure and the drain structure can determinebreakdown voltage. The distance of the lateral extension of the secondGaN layer beyond the field gate can be chosen accordingly.

It should be appreciated that the specific steps illustrated in FIG. 4provide a particular method of fabricating a lateral-drift or avertical-drift GaN FET according to an embodiment of the presentinvention. Other sequences of steps may also be performed according toalternative embodiments. For example, alternative embodiments of thepresent invention may perform the steps outlined above in a differentorder. Moreover, the individual steps illustrated in FIG. 4 may includemultiple sub-steps that may be performed in various sequences asappropriate to the individual step. Furthermore, additional steps may beadded or removed depending on the particular applications. One ofordinary skill in the art would recognize many variations,modifications, and alternatives.

Furthermore, although embodiments provided above, including FIG. 4, arediscussed in terms of GaN structures and layers, the present inventionis not limited to these particular binary III-V materials and isapplicable to a broader class of semiconductor materials with a wideband gap, including SiC and materials from the GaAs material group.Moreover, other III-V materials, in particular, III-nitride materials,are included within the scope of the present invention and can besubstituted not only for the illustrated GaN substrate, but also forother GaN-based layers and structures described herein. As examples,binary III-V (e.g., III-nitride) materials, ternary III-V (e.g.,III-nitride) materials such as InGaN and AlGaN, quaternary III-nitridematerials, such as AlInGaN, doped versions of these materials, and thelike are included within the scope of the present invention.

It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this applicationand scope of the appended claims.

What is claimed is:
 1. A lateral-drift GaN field-effect transistor(FET), comprising: a substrate; a first GaN layer coupled to thesubstrate, wherein the first GaN layer has a first conductivity type; asecond GaN layer coupled to the first GaN layer, wherein the second GaNlayer has a second conductivity type and comprises a void exposing aportion of the first GaN layer; a third GaN layer coupled to the secondGaN layer and to the exposed portion of the first GaN layer, wherein thethird GaN layer has a third conductivity type and comprises a voidexposing a portion of the second GaN layer; a source structure coupledto the third GaN layer, wherein a first portion of the third GaN layeris disposed between the source structure and the second GaN layer; adrain structure coupled to the third GaN layer; and a gate structurecoupled to the third GaN layer, wherein a second portion of the thirdGaN layer is disposed between the gate structure and the second GaNlayer.
 2. The FET of claim 1, further comprising a contact coupled tothe exposed portion of the second GaN layer.
 3. The FET of claim 1,wherein the second GaN layer comprises a plurality of sections that areprogressively thinner as a distance from the second GaN layer to thegate structure increases.
 4. The FET of claim 1, wherein the gatestructure comprises metal.
 5. The FET of claim 1, wherein the gatestructure comprises GaN of the second conductivity type.
 6. The FET ofclaim 1, wherein the first portion of the third GaN layer is thinnerthan a second portion of the third GaN layer.
 7. The FET of claim 1,wherein the third GaN layer comprises n-type GaN.
 8. The FET of claim 1,wherein the second GaN layer comprises p-type GaN.
 9. The FET of claim1, wherein at least one of the source structure, the drain structure,and the gate structure comprises a metal.
 10. The FET of claim 1,wherein at least one of the source structure, the drain structure, andthe gate structure comprises a metal and a semiconductor material. 11.The FET of claim 1, wherein a portion of the second GaN layer comprisesa field plate structure.
 12. The FET of claim 1, wherein the drainstructure does not overlap the second GaN layer.
 13. The FET of claim 1,wherein the second GaN layer extends from the gate structure toward thedrain structure by at least 1 μm.
 14. A vertical-drift GaN field-effecttransistor (FET), comprising: a substrate; a first GaN layer coupled tothe substrate, wherein the first GaN layer has a first conductivitytype; a second GaN layer coupled to the first GaN layer, wherein thesecond GaN layer has a second conductivity type and comprises a voidexposing a portion of the first GaN layer; a third GaN layer coupled tothe second GaN layer and the exposed portion of the first GaN layer,wherein the third GaN layer has a third conductivity type and comprisesa void exposing a portion of the second GaN layer; a source structurecoupled to the third GaN layer, wherein a first portion of the third GaNlayer is disposed between the source structure and the second GaN layer;a drain structure coupled to the substrate; and a gate structure coupledto the third GaN layer, wherein a second portion of the third GaN layeris disposed between the gate structure and the second GaN layer.
 15. TheFET of claim 14, wherein the second GaN layer comprises a plurality ofsections that are progressively thinner as a distance from the secondGaN layer to the gate structure increases.
 16. The FET of claim 14,wherein the first portion of the third GaN layer is thinner than thesecond portion of the third GaN layer.
 17. The FET of claim 14, whereina portion of the second GaN layer comprises a field plate structure. 18.The FET of claim 14, further comprising a contact structure coupled tothe second GaN-based layer.
 19. The FET of claim 14, wherein the drainstructure does not overlap the second GaN layer.
 20. The FET of claim14, wherein the second GaN layer extends from the gate structure towardthe drain structure by at least 1 μm.